An extrusion head of this kind is known from GB-PS 11 59 453.
For the production of sheathed elongated objects such as insulated tubes, sheathed cable, rubber cylinders, sheathed armored hose and the like, a plastic mass for the sheathing, usually a plastic or a rubber mixture, must be applied to the inner part of the product, the tube core, the profiled core, the cable core, the cylinder core or a tube. This is possible with extrusion heads through which the product to be sheathed is drawn and on which the plastic or rubber mixture is flowed by means of an extruder connected to the side of the extrusion head.
In carrying out this sheathing, there are two basic difficulties, namely the uniform flow of the inner portions for the purpose of obtaining a uniform wall thickness of the sheathing on the circumference and the homogenous flowing together of the two streams of material flowing around the core. The first problem is naturally greater when the sheathing material is difficult to work and still greater when the diameter of the sheathing and thus also of the extrusion head is greater. Especially in the sheathing of large cables and in the coating of cylinder cores with a rubber mixture, this problem is not easily solved. The second problem is still more serious since even with correct production procedure there occurs in the location where the two streams flow together an almost imperceptible inhomogeneity which impairs the quality of the end product in this location as, for example, limited strength, danger of bursting, flaws in the outer surface and the like. The reason for this lies in the unmistakably bad (molecular) union of the mass particles of the two streams of extrudant flowing together.
Improvement of the temperature control of the extrusion head is of as little help as the introduction of projections for pressing the portions of the mass together (DE-AS 12 45 100) or the use of a rotating ring which with its inner face engages the outer surface of the extrudant and smooths out the projections of the stream of extrudant (Ch-PS 347 345). But not only with a rotating outer ring as in Ch-PS 347 345 it has been sought to make the sheathing uniform also with a rotating core in the shear extrusion head which acts on the material forming the sheathing as described in the publication "Drahtwelt" 10-1979, page 416. This rotating core has a fully smooth outer surface. The action is intensified when, according to GB-PS 11 59 453 the rotating core is provided with a spiral. However here the above mentioned non-homogenous projections where the streams of extrudant flow together in the extrusion head are made helical so that these non-homogenous projections in a given length of the end product are longer than the projections running parallel to the axis. Hence the impairment of the product is increased.
Likewise in EP O 231 976 A 2 there is disclosed a rotating core with a screw which extends substantially to the tip of the rotating mandrel whereby the non-homogenous projections remaining in the product become still more distinct. Here there is provided on the rotating mandrel an axial packing between the stationary central tube and the rotating mandrel. Because of the high pressure forces, such axial packing is scarcely reliable and develops undesirable heat by friction which with polymerizable mixtures must be avoided.